Question: A parabola with equation  $y = x^2 + bx + c$ passes through the points $(2,3)$ and $(4,3)$. What is $c$?
Substitute $(2,3)$ and $(4,3)$ into the equation to give \[
3 = 4 + 2b + c \quad\text{and}\quad 3 = 16 + 4b + c.
\] Subtracting corresponding terms in these equations gives $0 = 12 + 2b$. So \[
b = -6\quad\text{and}\quad c = 3 -4 -2(-6) = \boxed{11}.
\]


OR


The parabola is symmetric about the vertical line through its vertex, and the points $(2,3)$ and $(4,3)$  have the same $y$-coordinate. The vertex has $x$-coordinate $(2+4)/2=3$, so  the equation has the form \[
y = (x-3)^2 + k
\] for some constant $k$. Since $y = 3$ when $x = 4$, we have $3 = 1^2 + k$ and $k=2$. Consequently the constant term $c$ is \[
(-3)^2 + k = 9 + 2 = 11.
\]



OR


The parabola is symmetric about the vertical line through its vertex, so the $x$-coordinate of the vertex is 3. Also, the coefficient  of $x^2$ is 1, so the parabola opens upward and the $y$-coordinate of the vertex is 2. We find $c$, the $y$-intercept of the graph by observing that the $y$-intercept occurs 3 units away horizontally from the vertex. On this interval the graph decreased by $3^2 = 9$ units hence the $y$-intercept is 9 units higher than the vertex, so $c = 9 + 2 = \boxed{11}.$